Hello Everyone,

I have been searching through this forum a discussion about PSV reaction force but unfortunately I can't find the specific one. If you do have a link please do send it to me.

My concern is PSV Reaction Force:

I am analysing an open system PSV directly above an equipment vessel by static analysis. I have referred to API RP520 for obtaining this force, what I'd like to understand is do we need to multiply this Force by 2 times for consideration of "dynamic load factor"? What is dynamic load factor and what part in the code can I read about it. By the way, I have read this in our project standard.

Thanks.

pinoy,

The dynamic load factor (DLF) is a characteristic of the applied load shape (your PSV force versus time curve) which is plotted vs. system natural frequency. It is determined by a Time History integration of the dynamic equations of motion. For impulse loads, such as your PSV, the maximum DLF is usually less than or equal to 2.0 (which is why this value shows up in your project standard).

To perform your PSV analysis statically, multiply your PSV thrust force by 2.0 (check your API RP520 standard as it may already be applying this DLF=2.0 factor) and apply this at your bend midpoint downstream of the PSV in your vent stack. The direction of the force is opposite the direction of fluid flow in your vent stack.

For an open system, if you have more than one bend in your vent stack then apply this force at each bend under a separate load vector.

For a closed system you would apply this force on bends on each “long” leg of pipe. The only way to truly figure out which pipe leg is short enough to ignore the PSV force is to run the force/time profile through Caesar II's DLF generator in the dynamics module, but then you might as well perform this analysis dynamically. For short pipes the duration of the unbalanced PSV force is small and this shifts the DLF peak to the right (higher frequency) which at some point is past the majority of your piping system natural frequencies of interest. But if you are going to do this statically you might simply take the nine longest pipe legs and apply your force to each bend corresponding to these longest legs. This would be the "brute force" approach, not really an approach based on physics.

You have 9 different force vectors to choose from so apply your PSV force under a different force vector for each bend because we want to only examine the effect on one bend at a time. Then set up separate OPE cases that include your different force vectors.

The OPE load cases should be used to determine your equipment and restraint loads. Subtract your standard OPE case from each of these, then add that result to SUS to obtain OCC code stress (note some codes use the OPE+F1 load case directly for comparison to the allowable stress, so how you need to review the particular code you are using to determine the proper approach here). Here is an example of load cases for B31.1 or B31.3 assuming 3 PSV forces, F1, F2, and F3, each applied at a different bend:

L1 = W+P1+T1 (OPE)
L2 = W+P1+T1+F1 (OPE)
L3 = W+P1+T1+F2 (OPE)
L4 = W+P1+T1+F3 (OPE)
L5 = W+P1 (SUS)
L6 = L1-L5 (EXP)
L7 = L2-L1 (OCC) segregated effect of F1
L8 = L3-L1 (OCC) segregated effect of F2
L9 = L4-L1 (OCC) segregated effect of F3
L10= L5+L7 (OCC) use Scalar Combination Method
L11= L5+L8 (OCC) scalar combination
L12= L5+L9 (OCC) scalar combination

Note that L10 through L12 are code compliance cases. L7 through L9 are interemediate load cases and not used for anything other than determining the stress results for L10 through L12.

Hello Loren,

Thanks for the quick reply, this is what I love about this forum... and thanks to COADE for making this possible.

Keep up the good work!

Hello Loren,

Thanks for your reply regarding PSV,I faced this problem while working on a project,I searched this form and got valuable information.

and thanks for Ianpinoy for asking this question.


with regards
ajay

Mr. Brown,

Do we really need to consider the temperature for stress evaluation of thrust force?

Mr.Loren,

You wrote:


My question is this example a case for Non-Liniear bondary condition?(because you take F from subtracting it with Basic OPE) and which Paragraph of B31.1 or B31.3? Para for Occasional Loads?

Dear Mr.Loren,

I want to ask, if you have 2 pumps running together and have to analysis the worst case, Pump A & B ON (T1), Pump A on Pump B off(T2), Pump B on Pump A Off(T3) and Pump A B OFF(T4) with design temp -39/45 deg c and Amb Temp -39/39 deg C. You have 2 piping spec here with Spec1 P=12 bar, Spec2 P=230 bar. This system has 3 PSVF1,F2,F3. Boundary condition Liniear.

So Please can you give an example how to build the case for PSV in Load case editor?
Because i confused, when T1 all 3 PSV are open, but when T2 it is only F1 and F3 will act, so does at T3 F2 and F3, for T4 it is only F3 act. I hope you understand my question.

Here i attached my example input for your review.

People,

When doing load cases, try to think about all of the reasonable and possible real life situations which could happen in the system.

whatever about the accademics, if your scenarios are over or under cautious, your wasting your time.


Example:
Some relief valves only open once at which you have to physically go and close it, therefore how can the reaction forces act on hot pipe? its impossible. (same as a Bursting Disk)

Why apply 3 valve reliefs at once? these things are quick and statistically, is it impossible for all 3 to open at exactly the same time?


So turn off the computer, get a pen, paper and a strong coffee and think about the design and operating regimes before pilling into caesar.

Hello Lauren
Could you please clarify as to what practical situation do these load cases refer to

L10= L5+L7 (OCC) use Scalar Combination Method
L11= L5+L8 (OCC) scalar combination
L12= L5+L9 (OCC) scalar combination

Dear Umair

L5+L7 is the sum of principal stresses in the system. You must be knowing that principal stresses are different from secondary stresses in a way that the pipe may not fail if the secondary stresses exceeds the limiting stress value(they are self limiting). But it can fail if the sum of principal stresses exceeds the limiting value.
now earthquake is a principal stress and so is the stress due to pipe wt(L5) hence they need to be added before they can be evaluated..

this is basic..asking Lauren for this is like asking God for coffee [:)]

Thank you for the reply manu ..the metaphor used in the end was pretty logical